This invention relates generally to fluid pressure actuated control devices and has to do more particularly with such a control device having sensitivity to and actuated in response to a change in pressure differential between low and high sides of a fluid system.
In my earlier U.S. Pat. No. 4,323,742, issued Apr. 6, 1982, I disclose a differential pressure switch incorporating several valuable features relating to high pressure operations, including shock resistance, corrosion prevention and ease of maintenance. The present invention has generally the same features but is characterized most particularly in three important aspects of improvement of the pressure sensing assembly;
(1) a much superior ability to respond quickly and without fail upon the occurrence of a change in the pressure differential;
(2) a much superior ability to accommodate and respond reliably to an extremely small pressure differential settings; and
(3) a substantially better sealing protection preventing leaking or blow-by of high pressure fluids through the sensing and actuating mechanism, either into the low pressure side of the fluid system or into contact with protected operating components of the switch assembly.
The switch operating assembly of the earlier patent includes a pressure sensing piston which travels axially within a cylinder bore in response to changes in pressure differential across the piston. The piston includes a flexible O-ring seal which engages the cylinder bore around the perimeter of the piston. The O-ring seal creates a resistance to axial movement which must be overcome before the piston will commence travel and also acts as brake during movement, both of which must be compensated for in the design of the actuating mechanism and limit to some extent the sensitivity of response of the piston to changes in pressure differential. In addition, repeated cyclings can result in a change in resistance to piston travel which can, in low pressure differential sensing, affect the reliability of the switch and require careful surveillance and maintenance.